Optical disk apparatuses that emit a light beam to an optical disk (e.g., a compact disc (CD), a digital versatile disc (DVD), or a Blu-ray disc (trade name, hereinafter referred to as a “BD”)) and read the light reflected by the optical disk so as to reconstruct information have been widely used.
Such existing optical disk apparatuses emit a light beam to the optical disk and locally change the reflectance of the optical disk. In this way, the optical disk apparatuses record information.
For such an optical disk, the size of a light beam spot formed on the optical disk is expressed as about λ/NA (λ: the wavelength of the light beam, NA: numerical aperture). It is known that the resolution is proportional to this value. For example, Non-Patent document 1 describes the details of a BD that can record 25-GB data on an optical disk and that has a diameter of 120 [nm].
In addition, optical disks are configured so that various content, such as music content and video content, or various types of computer data can be record thereon. In particular, in recent years, an amount of information of content has been increased in order to provide high-resolution video or high-quality music. Furthermore, the number of content items recorded in an optical disk needs to be increased. Accordingly, an increase in the capacity of an optical disk is required.
Accordingly, a method for increasing the capacity of an optical disk by stacking a plurality of recording layers in the optical disk has been proposed (refer to, for example, Non-Patent document 2).
In addition, as means for recording information on an optical disk, an optical disk apparatus using a hologram has been proposed (refer to, for example, Non-Patent document 3). Two types of such optical disk apparatuses are proposed: one is a method for recording information by locally generating holograms in an optical disk (called a positive-type method) and the other is a method for generating a hologram in advance by initializing an optical disk (hereinafter referred to as an “initial hologram”) and locally erasing the generated hologram (called a negative-type method).
For positive-type optical disk apparatuses, since a hologram needs to be generated in an optical disk when a user records information on the optical disk, a so-called two-side optical system that emits light beams onto either side of the optical disk is required. Accordingly, the structure is relatively complicated.
In contrast, for negative-type optical disk apparatuses, by simply erasing an initial hologram using high thermal energy and optical energy, a user can record information on the optical disk. Accordingly, a so-called one-side optical system that emits a light beam onto only one side of the optical disk can be used. As a result, the structure of the negative-type optical disk apparatuses can be simplified, as compared with that of the positive-type optical disk apparatuses.    Non-Patent Document 1: Y. Kasami, Y. Kuroda, K. Seo, O. Kawakubo, S. Takagawa, M. Ono, and M. Yamada, Jpn. J. Appl. Phys., 39, 756 (2000)    Non-Patent Document 2: I. Ichimura et al, Technical Digest of ISOM '04, pp 52, Oct. 11-15, 2005, Jeju Korea    Non-Patent Document 3: R. R. McLeod et al., “Microholographic multilayer optical disk data storage,” Appl. Opt., Vol. 44, 2005, pp 3197
Incidentally, as described above, for negative-type optical disk apparatuses, an initial hologram needs to be generated in an optical disk through initial processing in advance. In such initial processing, for example, by propagating plane waves so that the plane waves face each other, a volume type initial hologram can be formed. In this case, the initial processing takes, for example, about 5 [ms] until completion.
However, in order to emit plane waves onto an optical disk having a diameter of, for example, about 120 [mm], a laser light source having a high light intensity is required, for example. Consequently, the structure of an initializing apparatus for initializing an optical disk is large-scaled and complicated.
In addition, optical disks are required to maintain the formation states of a hologram and an initial hologram for day-to-day use. Accordingly, it is desirable that the material that forms a recording layer of an optical disk have a threshold value of sensitivity that does not respond to a relatively low light intensity and responds to a relatively high light intensity.
However, if optical disks have such a threshold value, the light intensity required for initializing the optical disks is increased. Accordingly, the initialization apparatuses require, for example, a laser beam source having a significantly high light intensity in order to initialize such optical disks. Thus, in practice, it is very difficult to produce such initialization apparatuses.